In the field of code division multiplexed data transmission systems such as is used in digital data transmission over hybrid fiber coaxial (HFC) cable systems and DOCSIS compatible data delivery systems in particular, there is a problem of narrow band interference. This problem is particularly found on the upstream transmissions in DOCSIS and other such non DOCSIS digital data delivery systems which deliver data over HFC (hereafter collectively referred to as DOCSIS systems). Because the upstream in a DOCSIS system is a noisy environment, there is a need to provide ingress cancellation systems at the head end of such DOCSIS systems to find and cancel or suppress narrowband interference.
One such prior art system is taught in U.S. Pat. No. 6,426,983 assigned to Terayon Communication Sytems, Inc. This patent teaches use of banks of near perfect analysis and reconstruction filters to break the spectrum into more narrow bands which are then analyzed individually for the presence of narrowband interference signals. Any band which is found to have narrowband interference present has its samples suppressed during the reconstruction. More precisely, the input signal is divided into a plurality of narrow frequency sub-bands using an analysis filter bank. Then the power in each sub band is monitored and compared to an adjustable or adaptive threshold in order to discriminate normal signal from narrowband interference. The signals that exceed the power threshold at the output of the sub bands are eliminated by replacing their sample values with zeroes). As the interference becomes lower and lower in power relative to the signal, completely eliminating the samples throws away a large amount of useful signal just to get rid of a small amount of interference. It would be better to weight each sub band based upon the amount of interferference in a sub band. Finally, the outputs of all sub bands are reassembled into the full band signal using a synthesis filter bank.
In the prior art U.S. Pat. No. 6,426,983, the analysis filters and the synthesis filters are implemented as FIR filter banks. More efficient narrowband interference cancellation requires that many narrow sub bands be used. This requires more FIR filters for the analysis and synthesis filter banks which drives the costs up. A more efficient implementation for the analysis and synthesis filter banks is needed. U.S. Pat. No. 5,838,268 teaches the use of Fast Fourier Transform circuitry to generate a plurality of separate carrier signals in a discrete multitone transmitter and inverse Fast Fourier Transform circuitry to receive the transmitted signal in the receiver.
FIG. 13 through FIG. 15 of the prior art U.S. Pat. No. 6,426,983 teach a predictor block circuit. This circuit is an adaptive FIR filter which functions to reduce any residual narrowband interference after the ICF block (ingress cancellation filter). The predictor block uses symbol decisions from the slicer to remove correlated (colored) noise in the slicer input. The predictor in code division multiple access (CDMA) systems operates after the despreader (in the code domain).
There is a need for a more efficient method of implementing the analysis and synthesis filter bank. There is also a need to improve the performance of the ingress cancellation filter by using optimal sub band weighting instead of censoring (setting all sample values in a sub band corrupted by narrowband interference to zero). There is also a need for a robust method for predictor functionality in CDMA systems.